Wireless communication services are an increasingly common form of communication, and demand for wireless services continues to grow. Examples of wireless services include cellular mobile telephones, wireless Internet service, wireless local area computer networks, satellite communication networks, satellite television, and multi-user paging systems. Unfortunately, these communication systems are confined to a limited frequency spectrum either by practical considerations or, as is often the case, by government regulation. As the maximum number of users, or “capacity,” of these systems is reached, user demand for more service may be met by either (1) allocating more frequency spectrum to the wireless service, or (2) using the allocated frequency spectrum more efficiently. Because the frequency spectrum is limited and cannot keep pace with user demand, there is a critical need for new technology that uses the allocated frequency spectrum more efficiently.
Wireless communication systems are generally composed of one or more base stations through which wireless users, such as mobile telephone users, gain access to a communications network, such as a telephone network. A base station serves a number of wireless users, fixed or mobile, in a local area. To increase the capacity of the systems, service providers may install more base stations, reducing the area and the number of users handled by each base station. This approach increases system capacity without allocating more spectrum frequency bands, but is quite costly because it requires significantly more hardware.
Another approach to using the frequency spectrum more efficiently is by improving “multiple access” techniques. Multiple access techniques allow multiple users to share the allocated frequency spectrum so that they do not interfere with each other. The most common multiple access schemes are Frequency-Division Multiple Access (FDMA), Time-Division Multiple Access (TDMA), Code-Division Multiple Access (CDMA), and more recently Space-Division Multiple Access (SDMA).
FDMA slices the allocated frequency band into multiple frequency channels. Each user transmits and receives signals on a different frequency channel to avoid interfering with the other users. When one user no longer requires the frequency channel assigned to it, the frequency channel is reassigned to another user.
With TDMA, users may share a common frequency channel, but each user uses the common frequency channel at a different time. In other words, each user is allocated a time slot in which the user may transmit and receive. Thus, TDMA interleaves multiple users in the available time slots.
CDMA allows multiple users to share a common frequency channel by using coded modulation schemes. CDMA assigns distinct codes to each of the multiple users. The user modulates its digital signal by a wideband coded pulse train based on its district code, and transmits the modulated coded signal. The base station detects the user's transmission by recognizing the modulated code.
In SDMA, a system may separate a desired user's signal from other signals by its direction of arrival, or spatial characteristics. This is sometimes referred to as “spatial filtering.” Thus, even though two users may be transmitting on the same frequency at the same time, the base station may distinguish them because the transmitted signals from the users are arriving from different directions. It is possible to use SDMA in combination with FDMA, TDMA, or CDMA.
A conventional SDMA receiver requires an array of multiple receive elements. Further, a conventional SDMA receiver uses a bank of phase shifters that cooperates with the receive element array to form a “beam” that “looks” in a particular direction. It is generally more desirable to form multiple beams, each directed toward a different direction, i.e., toward different users. The more beams, the more simultaneous users the SDMA system may handle operating on the same frequency at the same time. The more beams, however, the more complicated the SDMA receiver. For instance, each beam may require a separate bank of phase shifters and circuits that perform signal tracking. Additionally, each beam may require a separate “signal combiner,” which combines the signals received from each receive element to form a “combined signal.” Further still, each combined signal may require a separate signal detector, which detects the transmitted signal from the user. This hardware complexity greatly increases the cost of an SDMA receiver.
Using well known algorithms, hardware complexity may be reduced by performing phase shifting, signal tracking, signal combining and signal detecting in signal processing software. Current signal processing techniques, however, have difficulty identifying and tracking large numbers of simultaneously transmitted signals on the same frequency, particularly in a “multipathing” environment. A multipathing environment is one where transmitted signals may reach the receiver over multiple paths. For instance, a transmitted signal may reach the receiver (1) directly, and (2) indirectly after reflecting off objects. Multipath signals may also further complicate the complexity of the conventional SDMA receiver in the same manner as described above.
Thus, there is a need to provide an improved SDMA receiver that can simultaneously receive from multiple directions and operate in a multipath environment without likewise increasing hardware or software complexity of the receiver.